Electronic matrix arrays find considerable application in X-ray image sensors. Such devices generally include X and Y (or row and column) address lines which are horizontally and vertically spaced apart and cross at an angle to one another thereby forming a plurality of crossover points. Associated with each crossover point is an element (e.g. pixel) to be selectively addressed. These elements in many instances are memory cells or pixels of an electronically adjustable memory array or X-ray imager array.
Typically, a switching or isolation device such as a diode or thin film transistor (TFT) is associated with each array element or pixel. The isolation devices permit the individual pixels to be selectively addressed.
Amorphous silicon (a-Si) TFTs have found wide usage for isolation devices. Structurally, TFTs generally include substantially co-planar source and drain electrodes, a thin film semiconductor material (e.g. a-Si) disposed between the source and drain electrodes, and a gate electrode in proximity to the semiconductor but electrically insulated therefrom by a gate insulator. Current flow through the TFT between the source and drain is controlled by the application of voltage to the gate electrode. The voltage to the gate electrode produces an electric field which accumulates a charged region near the semiconductor-gate insulator interface. This charged region forms a current conducting channel in the semiconductor through which current is conducted. Thus, by controlling the voltage to the gate one may read or detect the charge detected by a storage capacitor or photodiode in a corresponding imager pixel. Herein, the TFT electrode which is connected to the pixel electrode or collector electrode is known as the "source."
Image sensors are known in the art. For example, see U.S. Pat. Nos. 5,498,880; 5,396,072; and 5,619,033, the disclosures of which are incorporated herein by reference. See also prior art FIG. 1.
Referring to prior art FIG. 1, a known image detector for capturing digital radiographic images is illustrated. The imager includes a plurality of pixels 3 each including a storage capacitor 7 and a switching thin film transistor (TFT) 5. The storage capacitor 7 in each pixel includes a collector electrode 9 which functions as a top plate of the storage capacitor, and a bottom plate 11 of the capacitor. Charge detectors 29 produce voltage outputs proportional to the charge detected on the corresponding Y address lines when gates are pulsed or energized. A more detailed description of how the FIG. 1 imager functions may be found in U.S. Pat. No. 5,498,880, the disclosure of which is incorporated herein by reference. See also U.S. Pat. Nos. 5,619,033; 4,672,454; 5,079,426; and 5,331,179 for other disclosures of known imagers including arrays of switching elements.
Unfortunately, the imagers of the above-listed patents are susceptible to at least one of the following problems. First, their imaging area is of a lesser area than would otherwise be desirable because the collector electrodes do not overlap the address lines and/or switching devices; or second, if such an overlap is disclosed, the imagers are plagued by undesirably high capacitive cross-talk in the areas of overlap, and the imagers require too many manufacturing steps to make. In view of these two problems, the TFT structure of commonly owned U.S. Pat. No. 5,641,974, which includes a photo-imageable low dielectric passivation layer covering each TFT, has recently been used in the manufacture of X-ray imagers. The insulation or passivation layer of this TFT structure is photo-imageable, in order to reduce the steps needed to make the imagers, and the low dielectric constant of the insulating layer reduces unwanted cross-talk (or noise in imagers). The insulating layer is located between the (1) address lines and TFTs, and (2) overlying electrodes, in order to insulate the electrodes from the address lines/TFTs in areas of overlap.
While this approach has been successful, it is susceptible to the following problem. Because the preferred insulating or passivation layer (e.g BCB or Fuji Clear.TM.) is substantially transparent to all wavelengths of light, including ultraviolet (UV) rays, UV rays used during the manufacturing process to expose or pattern the photoimageable insulating layer are often reflected back toward the imager by the stage of the stepper used in the manufacturing process. For example, when the insulating layer is of the negative resist type and is exposed to UV radiation during manufacturing by an i-line stepper using wavelengths of about 365 nm to pattern vias in the insulating layer, some of the UV radiation is transmitted through the insulating layer and imager and is then reflected by the stepper's stage back toward and through the same insulating layer. This type of reflection, at exposure energies of from about 200-300 mJ/cm.sup.2 for example, can cause mechanical features (or non-uniformities) in the stage (e.g. holes, shapes, grooves, imperfections, etc.) to be inadvertently imaged into the insulating layer, thereby resulting in such non-uniformities being visible in final X-ray images. This is undesirable. To reduce this effect, the exposure energy of the stepper can be decreased to about 100 mJ/cm.sup.2. Unfortunately, such reduction prevents sufficient cross-linking of the polymer insulating layer during exposure and causes partial removal of the polymer by the developer; and developer streaks and rings are likely to occur under such conditions. As a result, the process window is narrow when a photo-imageable polymer is used that has significant transmittance for UV light (i.e. the process and resulting product are susceptible to non-uniformities being inadvertently imaged into the polymer insulating layer).
It is apparent from the above that there exists a need in the art for an improved method for manufacturing a large area radiation imager including a photo-imageable insulating layer, wherein the adverse effect of UV rays emitted during manufacture are reduced so that inadvertent imaging of patterns or non-uniformities into the insulating layer is minimized. The method, and resulting product, should include a UV radiation protective (i.e. blocking/absorbing) layer in order to minimize the potential of the above-discussed problems.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.